The invention relates to a channel allocation method in a cellular radio network which comprises at least one base transceiver station and at least one subscriber terminal connected to the base transceiver station over a bi-directional radio link.
Further, the invention relates to a system for channel allocation in a cellular radio network which comprises at least one base transceiver station and at least one subscriber terminal connected to the base transceiver station over a bi-directional radio link and a serving cell.
One of the key problems in developing cellular radio networks is the limited scope of the available radio spectrum. The aim is to minimize the interference caused by a co-channel signal and an adjacent channel signal by carefully planning the use of radio frequencies. The frequencies are distributed according to various complex models into different cells with the aim to minimize the interference occurring in the radio links and thus maximize the network capacity. In the repetition pattern of a cell, the frequencies of the same or an adjacent channel cannot be too close to each other, because this causes too much interference in the system.
While the use of mobile phones and other subscriber terminals becomes more common, the capacity of networks must constantly be increased. This incurs high cost in form of frequency planning and various measurements.
In a dynamic channel allocation method of prior art, channel allocation is based on the average quality of the ongoing calls at a certain moment, i.e. the quality of each cell equals the average of all calls in the cell in question. The quality class of each cell is defined according to the average of the qualities of calls already connected. The quality class of a cell determines the probabilities to get the call through. Each probability value of channel reservation is between 0 and 1. The final probability P of reserving a channel is the product of the probabilities P(BTSn)=Pn of the adjacent cells 1 . . . n, i.e. the total probability P∈[0,1] is
xe2x80x83P=P(BTS1)* . . . *P(BTSn)=P1* . . . *Pn
The disadvantage of this prior art method is that in practice the decision on channel allocation is made based on the past situation, and no attempt is made to estimate the actual impact of the call to be set up on the cell. If the call is initiated very close to the serving base transceiver station or at the edge of the serving cell, all adjacent cells to the serving cell are not disturbed by it, only the adjacent cells closest to the call to be set up. According to prior art, the decision on the impact of interference is made on the basis of the quality of all ongoing calls in the adjacent cells. This emphasises the interference impact of a call to be set up too much, in particular when the call is initiated very close to the serving base transceiver station or at the edge of the serving cell. However, the call would probably not affect the quality of the calls of other cells if the subscriber terminal was very close to the serving base transceiver station, because both the uplink and the downlink would then have a low transmission power.
Thus, it is an object of the invention to develop a method and a system implementing the method in a manner that the above problems are solved. This is achieved by a method of the type described in the introduction, which is characterized in that a channel allocation decision takes into account the impact of a possible channel allocation on the interference level of the cellular radio network, and by a channel allocation system which is characterized in that the serving cell is adapted to allocate channels by estimating the impact of a possible channel allocation on the interference level of the cellular radio network.
The preferred embodiments of the invention are set forth in the dependent claims.
The invention is based on the idea that the decision on channel allocation, i.e. the allocation of a channel for a call, is also at least partly based on an estimate of the actual impact of the call on the interference level of the radio cellular network at least in the immediate surroundings of the serving cell. By estimating the actual impact of each call on the interference level, it is possible to avoid the problem of prior art method to reject a call only because calls in other cells using the same frequencies as the serving cell have a low quality, even though the call would, for instance due to the location of the mobile station in the radio environment, probably not at all reduce the quality of the calls in the other cells. Because the method of the invention tries to estimate the actual impact of the call on the total quality in the cellular network, the call can be accepted regardless of the current quality or interference level in the other cells, if the estimated impact of the call to be set up on the interference level or quality is low.
The interference impact of an incoming call varies considerably depending on where the mobile station is in relation to the serving base transceiver station and the cells which will possibly suffer from interference. If the mobile station is close to the serving base transceiver station, it is probable that the transmission power in both the uplink and the downlink is low and the call will have no major impact on the quality of calls in other cells. If the mobile station is close to the edge of the serving cell, it is probable that it has a greater impact on the quality of the calls in adjacent cells on this side than on the adjacent cells on the other side of the serving cell. In an embodiment of the invention, the estimated impact of a call on individual adjacent cells is weighted in a different way depending on the location of the mobile station in the radio environment, and thus the impact of the radio environment location of the mobile station on the acceptance or rejection probability is taken into account. Thus, in practice, the location of a mobile station in the radio environment determines how well it receives signals from the different cells.
In the first preferred embodiment of the invention, the radio environment location of a mobile station is taken into account in the channel allocation decision by examining the received downlink signal strengths of both the serving cell and the adjacent cells, which are measured in the mobile station. If the signal strength received from an adjacent cell is high, it can be assumed that the impact of a transmission of the mobile station on the adjacent cell in question is also high. If the level of the signal received from an adjacent cell is low, it can be assumed that the interference impact of the mobile station on the adjacent cell in question is also low. Thus, the estimate on the interference impact of an incoming call must increase when the signal strength received from an adjacent cell increases and, correspondingly, decrease when the signal strength decreases. Correspondingly, the estimate on the interference impact on the adjacent cells must decrease when the serving cell signal strength increases and increase when the serving cell signal strength decreases. By combining the estimated impact on individual adjacent cells and the impact estimated on the basis of the serving cell, an estimated impact of an incoming call on the total interference level or quality is obtained and a decision to allocate a channel or reject the call can be made on the basis of this.
The method and system of the invention provide several advantages. A great advantage is that the capacity of the cellular network increases with an efficient reuse of frequencies which is achieved by an efficient selection of channels.
The method of the invention takes into account better than prior art the location in the radio environment of the subscriber terminal accessing the network before the network makes a decision on accepting or rejecting the call. This is done by monitoring the strengths of the received signals of the serving cell and the adjacent cells. With the method of the invention, it is possible to find out the actual cells suffering from interference and the actual interference caused by the call to be set up. The method of the invention efficiently utilizes both the already collected information as a basis for decision-making and the estimate on the impact of the new call. Thus, this advanced interference control method allows a better and more efficient utilization of a cellular network capacity. The method of the invention also reduces quality problems when the subscriber terminal requesting the service is close to the serving base transceiver station. This occurs when the interference impact on the adjacent cells of the call to be set up is estimated to be low. In such a case, the call can be connected to the serving cell regardless of the quality of the adjacent cells, and an unnecessary call rejection is avoided.
The system of the invention provides the same advantages as, described above for the method. It is obvious that the preferred embodiments and specific embodiments can be combined in various combinations to provide the desired technical power.